1. Field of the Invention
The present invention relates to image forming apparatus, such as a printer, a copier, and the like, particularly to a belt conveyor having the function of correcting meandering of endless belts, such as an intermediate transfer belt, a sheet transfer belt, and the like, and an image forming apparatus using the belt conveyor.
2. Description of the Related Art
In relation to a multicolor image forming apparatus, such as a full-color printer or a spot-color printer, a tandem-type multicolor image forming apparatus is available. In this system, a plurality of photosensitive drums are arranged along a conveying direction of an intermediate transfer belt, which is an endless belt, and toner of different colors is caused to adhere to electrostatic latent images formed on the respective photosensitive drums, to thus form toner images and sequentially transfer the toner images on the transfer belt.
This type of the apparatus inevitably encounters a phenomenon of an intermediate transfer belt, which is an endless belt, moving in a width direction thereof in association with driving of the intermediate transfer belt, i.e., a meandering phenomenon of the belt. This meandering phenomenon causes positional offsets of color images and, by extension, color misregistration, when the images of respective colors are transferred onto the intermediate transfer belt in a superposing manner. Therefore, the meandering phenomenon must be corrected.
There are several methods for correcting meandering of the transfer belt. One of them is a method for taking one of rollers supporting a transfer belt as a meandering correction roller and controlling the inclination of the meandering correction roller.
FIGS. 14A to 14C are descriptive views illustrating the control method. When one side edge of a correction roller 20 is raised from a state shown in FIG. 14A to a state shown in FIG. 14B, the transfer belt shifts toward the side edge of the raised side of the roller. In contrast, when one side edge of the correction roller 20 is lowered as shown in FIG. 14C, the transfer belt shifts in a direction opposite to the lowered side of the correction roller. Accordingly, the amount of shift of the transfer belt can be controlled by means of varying the inclination of one side of the correction roller 20 with respect to the other side.
One technical problem encountered by the method for controlling the inclination of the meandering correction roller is a method for detecting the amount of meandering of the transfer belt over a wide range and with a high degree of accuracy. Another problem is to detect an anomaly when the amount of meandering has exceeded a certain range, to thus prevent occurrence of breakage of the belt without fail. The respective technical problems will be described hereunder.
A system disclosed in, e.g., JP-A-2000-034031, has been known as a method for detecting movement of an endless transfer belt in the width direction thereof, i.e., meandering.
As shown in FIG. 15, this method is achieved by means of placing a contact 52 at the side edge of the transfer belt 51; supporting the contact 52 so as to be rotatable around a support shaft 53; causing one member 52a of the contract 52 to keep contact with the transfer belt 51 at all times by means of tensile force of a spring 54; and arranging a displacement sensor 55 in close proximity to another member 52b. The displacement sensor 55 includes, e.g., alight-emitting section and a light-receiving section. The light emitted from the light-emitting section is reflected from an object of measurement, to thus detect a distance between the object of measurement and the displacement sensor 55 from the position of reflected light received by the light-receiving section and displacement of the reference position.
According to such a configuration, when the transfer belt 51 has caused meandering, the contact 52 rotates around the support shaft 53 in association with the meandering, whereby the distance between the member 52b and the displacement sensor 55 is displaced. Accordingly, the amount of displacement is detected by the displacement sensor 55, so that the amount of displacement of the transfer belt 51 in the width direction can be detected.
The amounts of meandering that can be detected by the system; that is, the amount of displacement of the transfer belt 51 in the width direction, is determined by a distance Y2 between the support shaft 53 and the transfer belt 51 and a distance Y1 between the support shaft 53 and a point of measurement of the displacement sensor 55
Provided that a detection range of the displacement sensor 55 is taken as 10 mm, in the case of Y1=Y2, the amount of detectable displacement of the transfer belt 51 in the width direction assumes 10 mm. In this case, the detection accuracy of the amount of displacement of the transfer belt 51 becomes equal to that of the displacement sensor 55, because Y1 and Y2 assume a proportion of 1:1.
In order to increase the amount of detectable displacement of the transfer belt 51, the proportion between Y1 and Y2 (Y1/Y2) is assumed to be ½, the amount of detectable displacement of the transfer belt 51 comes to 20 mm. In contrast, the detection accuracy of the position of the edge of the transfer belt 51 becomes half the accuracy of detection of the displacement sensor 55.
Accordingly, when the displacement sensor 55 is used for detecting the position of the edge of the belt 51, the distances Y2 and Y1 are appropriately selected such that the range of displacement of the belt 51 in the width direction falls within the detectable range of the displacement sensor 55. For instance, when the range of displacement of the belt 51 is of the order of 5 mm, the detection range of the displacement sensor 55 is usually 2 mm or thereabout. Hence, the range of displacement of the belt 51 is caused to fall within the detection range of the displacement sensor 55 by means of making the distance Y1 be greater than the distance Y2.
However, in order to lessen positional displacements of the toner images of respective colors in an image forming apparatus, the amount of displacement (meandering) of the belt 51 in the width direction must be detected with high accuracy, to thus correct the meandering of the belt 51. For this reason, the proportion between Y1 and Y2 is desirably made close to or equal to 1:1. However, according to the above method, the range where movement of the transfer belt can be detected and the detection accuracies are contrary to each other. Hence, difficulty is countered in detecting the displacement over a wide range and with a high degree of accuracy.
The second technical problem is to detect anomalies in meandering of the transfer belt. When the anomalies, such as meandering of the transfer belt exceeding the detectable range of a displacement sensor arises, driving of the belt must be stopped, to thus prevent occurrence of fracture of the belt.
JP-A-Hei. 6-9096, U.S. Pat. No. 5,784,676 and JP-A-2001-130779 provide several proposed methods for addressing anomalies when the meandering of the transfer belt increases. In general, when the displacement sensor detects an anomaly, a signal is input to a microprocessor. The microprocessor controls a drive roller of the transfer belt so as to stop the drive roller. However, in order to reliably prevent occurrence of an accident, such as fracture of a belt, realization of a highly-reliable anomaly detection system has been desired.